Before we all start jumping to comclusions why don't you answer a few simple question.
1. What suspension are you running?
2. have you swapped in a different rear?
3. Do you know what brakes you have on the front? how about the rear?
If you have a stock or slightly modyfied mustang, you can go right here: http://www.streetortrack.com/torq-thrust-II.asp
and order the 17x8 wheels with 4.75 Backspacing and have them in a couple of days.
If for somereason your car has the incorrect rear end or wilwood disc brakes they would affect your track...more than likely though, the store you bought the wheels from gave you a story
and the wheels were just wrong for your application.